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Investments - Narrative (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]          
Debt Securities, Available-for-sale, Amortized Cost $ 1,692,587,000   $ 1,692,587,000   $ 1,358,960,000
Cumulative net unamortized unrealized gain $ 28,000   $ 28,000   $ 15,247,000
Maximum percentage of shareholders' equity security holdings held of one issuer 10.00%   10.00%   10.00%
Securities pledged $ 596,100,000   $ 596,100,000   $ 752,700,000
Mortgage-backed securities, weighted average life     5 years    
Allowance for credit losses on available-for-sale securities 0   $ 0   0
Accrued interest receivable, held-to-maturity securities 5,200,000   5,200,000   3,400,000
Allowance for credit losses on held-to-maturity debt securities (689,000)   (689,000)   (614,000)
Sale of debt securities, available-for-sale 0 $ 27,100,000 0 $ 27,135,000  
Other investments, at cost 58,637,000   58,637,000   59,896,000
Reclassified to Held to Maturity          
Debt Securities, Available-for-sale [Line Items]          
Debt Securities, Available-for-sale, Amortized Cost 116,900,000   116,900,000    
Cumulative net unamortized unrealized gain 18,500,000   18,500,000   21,000,000
Des Moines          
Debt Securities, Available-for-sale [Line Items]          
Other investments, at cost $ 10,000,000   $ 10,000,000   $ 12,100,000